Magnetic focusing arrangement for cathode ray tubes



Patented Aug., 4 1942/ srs ru o Y l'2,291,682 vica MAGNETHC FOCUSING GEMENT FOR CATHODE RAY TUBES application august s, 194e, semi ne. 351,592

lin Great Britain July 28, 1939 ('Ci. Z50-27) 7 iliaims.

tubes employing very high voltages as in thecase ci' projection tubes used in television receivers in cinemas.

The electron beam in a cathode ray tube employing magnetic focusing is liable to become de-l iocused if the voltage applied to the final anode ci the tube should alter or if the magnetic focusing current should alter due to iuctuations in the main supply. Defocusing will also occur if the anode voltage alters due to regulation of the anode supply or again ii. the focusing current alters duc to temperature changes in the magnetic iccusing coil.A

'it is common to employ stabilising arrangements 'for the high tension voltages supplied to thermionic valves and cathode ray tubes and the shown stabilising methods could be adapted to overcome all the undesirable effects referred to but for the fact that very much higher voltages are used in connection with the type of cathode ray tube with which the invention is more particularly concerned, and it would be inconvenient to develop the stabilisirrg arrangements which would become necessary.

The object of the present invention is to provide a method and apparatus for overcoming defocusing in a magnetically focused cathode ray tube Without the requirement of special stabilising arrangements.

According to the present invention in a circuit arrangement including a cathode ray tube, a source of operating potentials for said tube and electromagnetic means ior focusing the electron beam thereof, eiocusing of said electron beam due to variation in the resistance of the focusing coil or due 'to variation in the voltage applied to an anode of the tube lis reduced or elimiruitedI duces a certain percentage change of focus current, and the ratio of these two percentages is fixed in accordance with the degree of saturation of any iron associated with the focusing coil. Further, the focus current may be varied in order that adjustment t obtain an optimum focus may be eiected without substantial modification of said percentage ratio.

In a particular arrangement according to the invention the focusing coil is connected in the anode circuit of a pentode type valve and variations in potential oi the anode of the cathode ray tube are applied simultaneously to the control grid of the pentode valve. A potentiometer arrangement may be employed to transmit the anode potential variations to the control grid oi the valve and in one modication, a cathode follower valve isinterposed between the potentiometer and the pentode or other valve.

In order that the invention may be more clearly understood and readily carried into effect, two fragmentary circuit arrangements for carrying out the invention will now be described in greater detail by way oi example with reference to the accompanying drawing. In the `drawing Figure 1 shows a circuit arrangement ior maintaining the desired relationship between the voltage of the anode of the tube and the coil current and simultaneously forpreventing alteration or iocusing current through temperature changes in the coil, while Figure 2 shows a modication of the arrangement of Figure 1.

It is known that, when a voltage V is applied between anode and cathode oi a cathode ray tube and the focusing eld is provided by a current l in a suitable coil, the relation between V and I to maintain complete focus is of the form L=KV% in the absence or magnetic saturation if the coil is provided with an iron yoke. For small variations of the order it is required to correct, this gives 1 I A= VAV (1) In practice with iron clad coils magnetic saturas tion does occur to some extent and it is found experimentally that this affects the value of the fractional co-efdcients of I v AV For instance, in the case of a coil oi practical construction the relation is I AI=.64VAV (2) if the expression is re-Written as I Av' Ari-Mv it is obvious that for maintenance of focus the fractional change of I should be .64 multiplied by the fractional change of V.

Referring to the drawing, i represents the focusing coil for a cathode ray tube which is connected in the anode lead of a pentode valve 2, anode current being derived from a source (not shown) to which the terminal 3 is connected. The screening grid ofthe valve 2 is connected at terminal li to a source of stable voltage (not shown), and the cathode 5 is connected to earth through a resistance 6. The control grid of the valve 2 is connected on one hand through a resistance 1 to a known fraction of the cathode ray tube anode potential at the terminal l and 5 on the other hand through a resistance 9 to a source of stable voltage at the terminal Il). The stable voltage may be provided by a battery or an articially stabilised mains rectifier source or a source rectified from astable alternating current supply which in general is liable to percentage variations which are small compared with the percentage variations to be expected inthe high voltage supply feeding the cathode ray tube anode. It will be understood that as the valve 2 is of the pentode type and therefore has a high anode impedance, the current througlrthe coil i is substantially independent of small fluctuations of its resistance such as might be caused by temperature variations, the current also being independent of small variations in the potential of the source to which the terminal 8 is connected.

If I is the current through the coil, then neglecting screen current. the potential at the grid of valve 2 is approximately:

Allowing for screen current and the finite voltage ampliiication factor between grid and screen. a similar linear equation is obtained with modined constants.-

'lhe potential o at the grid of the valve is made up of a component o', which is xed, being derived from the stabilised source 4 and a variable component, a fraction of the anode supply of the cagiade ray tube, say V/a.

The change of current corresponding to a small' change of V, AV is therefore Substituting the value nl AV from Equation 2 it will be seen that 60 (Tf) By substituting in Equation 4 65 V V #fum-v .alf 7o =0.64 (5) zvr'iviri'v' which is the condition required for keeping the tube focused.

In order to provide focusing current without altering the relation expressed in the Equation 5, a cathode follower valve Il, a variable .potentiometer if and resistance I 3, large compared with the inverse of the slope of valve Il. connected to a stabilised source of supply'at the terminal Il have been added .to the arrangement of Figure l. The anode of the valve Il is connected to a stable source of supply at the terminal 4. The potential at the terminal l0 is modified to provide a standing bias for the control grid of the valve il. Under these conditions the potential at the cathode of the valve Il is very nearly V/a+v' with respect to earth.Y The potential at the terminal Il is made equal to o' so that the potential at the cathode with respect to that at the terminal For a given position of the-tapping point on the potentiometer the potential at the grid of the valve 2 with respect to that at the terminal It is a fraction. say b of V/a+o'+v1, `and the potential with respect to earth is therefore so that. the condition for focus compensation is as in the circuit of Figure 1 and is independent of b. Thus, the potential at the grid of the valve 2 and hence the current flowing through the focusing coil. can be varied by moving the slider of the potentiometer l2 without altering the law of the compensation.

We claim:

1. In a cathode ray system wherein is provided a cathode ray tubehaving an electron emitting cathode and an anode for accelerating electrons emitted from said cathode land wherein a magnetic coil is supplied for focusing said electrons into a beam, the method of maintaining focus in spite of spurious fluctuations of anode potential and coil current supply, which comprises feeding current to said coil through a constant current impedance path, adjusting said current whence to a predetermined value, and altering said predetermined value of current in accordance withA into a beam. the method of maintaining focus in spite of spurious fluctuations of anode potenmeans `for varying the tial and coil current supply, which comprises feeding current to said coil through a constant current impedance path, adjusting said current to a predetermined value, and altering said predetermined Value of current in proportion to iluc tuatlons of the anode potential from a prcdetermined value of voltage.

3. A cathode ray system comprising a cathode ray tube having at least an electron emitting cathode and an electron accelerating anode, a magnetic focusing coil positioned intermediate said cathode and said anode, means to supply electrical current to said coil through a constant current impedance, means for adjusting said current to a predetermined value, and means for altering said predetermined value of current in accordance with fluctuations from a predetermined value of voltage of potential supplied to said anode.

4. A cathode ray system comprising a cathode ray tube having at least an electron emitting cathode and an electron accelerating anode, a magnetic focusing coil positioned intermediate said cathode and said anode, means to supply electrical current to said coill through a constant current impedance, means for adjusting said current to a predetermined value, and means for altering said predetermined value of current in proportion to fluctuations from a predetermined value of voltage of potential supplied to said anode.

5. A cathode ray system comprising a cathode ray tube having at least an electron emitting cathode and an electron accelerating anode, a magnetic focusing coil having a saturable iron core positioned intermediate said cathode and said anode, means to supply electrical current to said coil through a constant current impedance,

means for adjusting said current to a predetermined value, and means for altering said predetermined value of current in accordance with fluctuations from a predetermined value of voltage of potential supplied to said anode in accordance with the degree of saturation and uctuations of potentials supplied to the anode from a predetermined value of voltage.

6. A cathode ray system comprising a cathode ray tube having at least an electron emitting cathode and an electron accelerating anode, a magnetic focusing coil positioned intermediate said cathode and said anode, means to supply electrical current to said coil through a thermionic tube having substantially constant anode current, means for adjusting said current to a predetermined value, and means for altering said predetermined value of current in accordance with fluctuations from a predetermined value of voltage of potential supplied to said anode.

7. A cathode ray system comprising a cathode ray tube having at least an electron emitting cathode and an electron accelerating anode, a magnetic focusing coil positionedy intermediate said cathode and said anode, means to supply electrical current to said coil through a thermionic tube having substantially constant anode current, means for adjusting said current to a predetermined value, and electrical bridge means for altering said predetermined value of current in accordance with uctuations from a predetermined value of voltage of potential supplied to said anode.

ALAN DOWER BLUMLEIN. FRANK BLYTHEN. 

